1. Field of the Invention
The present invention relates to an improvement in heat management and process control for a molding process and, more particularly, to the use of active and/or passive film heating and/or sensing elements located along a flow channel of molten resin to a mold cavity space.
2. Description of the Related Art
In an injection molding process, it is important to maintain a resin in a molten state as it flows from a nozzle of an injection molding machine, through a mold sprue bushing, a mold manifold, a hot runner nozzle, and into a mold cavity space, where the resin cools to form an injection-molded article. Additionally, the shear stress profile of the flow of resin must be monitored and managed to insure proper filling of the cavity space. This is especially important in the area close to the mold gate because the temperature there is rapidly cycled between hot and cold conditions before the molded article is removed from the cavity. Temperature control issues are also very important when molding certain thermally-sensitive materials such as PET in a multicavity mold or when molding articles made of different materials that are injected through a single hot runner nozzle. Accordingly, much effort has been directed towards improving heat management and process control in the injection molding process, particularly in the mold manifold and hot runner nozzle. To date, several methods and means have been employed with varying degrees of success. Included among the methods and means commonly employed are heat pipes, high frequency induction heaters, microwave heaters, ceramic heaters, infrared radiation heaters, electrical heaters, etc. Such electric heaters include coils, band, or cartridge heaters which are used to heat the molten resin inside the screw barrel, in the machine nozzle, in the manifold, in the hot runner nozzle, and in the mold gate area.
U.S. Pat. No. 5,645,867 issued to Crank, et al. (incorporated herein by reference) illustrates the current state of the art with respect to heating the mold manifold. Crank, et al. teaches heating the manifold by disposing infrared radiation heaters on an outer surface of the manifold. However, as is typical of such prior art manifold heating apparatuses, a significant proportion of the heat generated by the heaters is wasted heating the entire manifold block rather than directly heating the resin flowing in a melt channel contained therein.
U.S. Pat. No. 5,614,233 issued to Gellert (incorporated herein by reference) discloses a state of the art heater for a hot runner nozzle, in which a helical electrical heater is embedded in a spiral groove that surrounds the hot runner nozzle. The heater comprises a resistive wire enclosed in a refractory powder electrical insulating material such as magnesium powder oxide. The helical portion of the heater is press-fitted and reshaped into place in the spiral groove. However, the disclosed heater heats both the hot runner nozzle body and the melt channel contained therein, a relatively inefficient heating arrangement. Additionally, manufacturing the spiral groove and assembling the heater therein is time-consuming and costly.
The foregoing problems with prior art heaters are particularly evident in coinjection and multiinjection mold manifolds and hot runner nozzles. For example, U.S. Pat. No. 4,863,665 issued to Schad, et al. (incorporated herein by reference) discloses the use of a single electrical heater attached to the outer surface of a hot runner nozzle to heat three melt channels simultaneously. Schad, et al., however, faces several drawbacks. First, less heat is transmitted to the inner channels than to the outer channels. Second, the heat supplied to each channel cannot be varied according to the size of each channel and the rheological characteristics of the resin flowing therein.
European Patent 312 029 B1 issued to Hiroyoshi (incorporated herein by reference) discloses a heater made of an insulating ceramic film that is flame-sprayed on the outer surface of the nozzle which introduces the resin into the molding machine. The heater may be a continuous area heater completely covering the nozzle, a heater made of a plurality of longitudinal strips, a thin film heater made of helical strips, or a two piece independent heater with more power supplied to the nozzle where it contacts the mold. However, the heater disclosed in Hiroyoshi has several significant drawbacks that militate against its application to a mold manifold or hot runner nozzle. First, the Hiroyoshi heater is not removable and thus requires replacement of the entire element when the heater burns out. Second, the heater inefficiently heats the entire machine nozzle body rather than directly heating the molten resin. Third, the heater cannot provide a profiled temperature gradient across the flow of molten resin, an important feature for managing shear stress in the flow of molten resin. Finally, the thickness of the disclosed heater is 0.5 to 2 mm, which is acceptable for application to the outer surface of the machine nozzle, but intolerable for application to the interior of a melt channel in a mold manifold or hot runner nozzle.
U.S. Pat. Nos. 5,007,818 and 5,705,793 disclose the use of heaters which are deposited directly on the flat surface of the cavity mold. U.S. Pat. No. 5,504,304 discloses a removable ceramic heater made of a ceramic paste whose thickness is hard to control. Such heaters as these do not provide for intimate contact with the nozzle body or the nozzle tip and thus reduce heat transfer and increase heat loss. Reference also made be had to the following U.S. patents (each of which is incorporated herein by reference) which disclose heater technology; U.S. Pat. Nos. 5,155,340; 5,488,350; 4,724,304; 5,573,692; 5,569,398; 4,739,657; 4,882,203; 4,999,049; and 5,340,702.
Accordingly, there is a need in the art for a method and means of heating a melt channel of a mold manifold and hot runner nozzle in a manner that is efficient in terms of energy, space, and location.
There is an additional need in the art for an efficient method and means of providing an appropriate amount of heat to each melt channel in a coinjection or multiinjection hot runner nozzle based on the localized size and shape of each melt channel and the rheological characteristics of the resin flowing therein.